Modern guns, sure. Older firearms, such as the rifles used by samurai in the 16th century, would be much less effective, and probably more suitable for the setting as well.

Yikes, Gharnef can only be beaten by one spell as is. To give him a gun would be trollsome. But then again, he chased Marth's army across the desert while being invulnerable, so he is already a bit of a troll. xD